Many people enjoy watching television, and the television-watching experience has been greatly improved due to HDTV. Although a great number of people pay for HDTV through their existing cable or satellite TV service provider, in fact, HDTV signals are required to be broadcast over the free public airwaves. This means that HDTV signals may be received for free with the appropriate antenna.
Modern homes often have several TV sets located in multiple rooms, for example, in living rooms, bedrooms and family rooms, where different individuals may be simultaneously watching dissimilar TV program channels. Such homes have TV signal distribution wiring which typically carries signals from a single antenna location and distributes them to each set location.
However, often the signals for different channel frequencies arrive at the antenna from different transmitter directions. Available antennas are usually optimized for highest signal sensitivity, meaning that they have relatively narrow acceptance angles, so pointing the antenna for best reception of one channel might not be optimum for receiving another channel. This creates a problem when the same antenna is to be used for receiving more than one channel at a time.
In the past, the direction problem has been addressed by using a) a motorized antenna rotator to aim the antenna toward the broadcast being watched, b) an array of antennas, each aimed toward different arriving signals, or c) an omni-directional antenna. The first solution does not solve the multiple-viewer issue and adds cost and inconvenience; the second solution requires that the antennas in the array be at least one wavelength apart at the lowest frequency and have electrical isolation, i.e. separate amplifiers or filters associated with each antenna in the array, the result of which is both bulky and expensive; and the third solution is mainly applicable in close proximity to the TV transmitters because omni-directional antennas generally have low signal sensitivity and no directions from which the antenna can reject interfering signals.
Various antennas and/or antenna/reflector combinations, such as wideband antennas and bow-tie antennas, are disclosed in U.S. Pat. Nos. 2,918,672, 3,373,432, 4,160,980, 4,293,861, 6,466,178, 6,480,168, 7,050,013, 7,990,332, 8,674,897, 8,773,322, 8,994,600, 9,281,566.
Most HDTV digital signals are broadcast in the high UHF band from 470 to 710 MHz. Therefore, a need exists to provide a compact UHF antenna optimized to receive high definition television (HDTV) digital signals in the UHF band. A further need exists for a wide beam width antenna with high interference rejection. Yet a further need exists for a UHF antenna with good sensitivity. Another need exists for a low cost HDTV antenna for use outdoors or indoors that has an aesthetic appearance. Furthermore, a need exists for such an antenna to be easily constructed for low cost using conventional manufacturing methods and materials.
Many antennas are based upon the dipole principle. Starting with a basic dipole element, many antenna designs add passive elements, such as directors and/or reflectors, to enhance their performance in order to achieve certain performance goals. For example, the well-known Yagi-Uda antenna design employs this method to enhance the sensitivity in a desired direction, at the expense of sensitivity in other directions. In other words, the Yagi antenna has good gain, but is highly directional. The use of added passive elements is particularly true of terrestrial television (TV) antenna designs where the extra elements help to improve their signal capturing gain, but also where this method limits the ability of such antennas to simultaneously receive signals from multiple directions.
Since in North America, terrestrial TV signals are required to be horizontally polarized, i.e. the e-field lies in the horizontal plane, TV antennae must be designed to pick-up such signals. The basic horizontal dipole antenna is horizontally polarized, so it has an antenna pattern which is omni-directional in the vertical plane, but has a figure-8 shape in the azimuthal (horizontal) plane. Thus, the dipole has maximum sensitivity when the signal approaches from the broad side of the antenna, but no pick-up sensitivity when the incoming signal approaches from the ends of the antenna.
Antenna gain is specified as the ratio of sensitivity in the direction of greatest signal pick-up to that of a reference antenna, expressed in decibels (dB). The reference antenna is commonly taken as a hypothetical intrinsic antenna—one with equal sensitivity in all directions—so the gain is expressed in dBi, where the “i” indicates that the reference being used is the intrinsic antenna. In the direction of maximum sensitivity, i.e. when the incoming signal approaches broadside to the antenna, the dipole has a (maximum) gain of about 2 dBi.
In turn, the half power beamwidth (HPBW or 3 dB down BW) is approximately 70 degrees. Note that at the edges of the beam, the gain is −1 dBi which is actually less than that of the intrinsic antenna. Thus, the low gain and narrow beamwidth of the unaided dipole makes it a poor candidate for TV reception, in spite of its wide use in the common “rabbit ears” design.
One approach to improving the gain of a dipole antenna is to add a passive reflector behind the active dipole in order to redirect incoming energy, which has not been captured and has passed by the dipole, back toward the dipole. Such reflectors are highly effective. An extreme example is the parabolic dish antenna used for radio astronomy or spacecraft communications, where many tens of dBi of gain are achieved, yet still using only a dipole as the active element. However, whenever a reflector is used to improve gain, there is a sacrifice in beamwidth. The above mentioned dish antennas have extremely narrow beamwidths, which is beneficial for radio astronomy or space communications, but not necessarily for terrestrial TV reception. When adding even a simple plane reflector behind a dipole, the maximum forward gain can be increased to over 7 dBi, but the beamwidth then decreases to less than 65 degrees.
In virtually all terrestrial TV reception antenna designs, both halves of the dipole are energized by the same electromagnetic (EM) wave. Even with a reflector, where the dipole receives energy from two directions at once, there is no difference between the energy arriving at each half of the dipole. This is also true when directors are added to the design. The active dipole element “sees” the same signal energy and amplitude at each half, and in-phase, as is required for proper operation.
FIG. 1 shows the signal excitation pattern for a simplified conventional TV antenna 10 design employing a dipole active element 12a, 12b with a passive reflector 14. The thin solid lines designate the arriving EM wave with the direction of arrival shown by the solid hollow arrow 16. The dashed lines designate the reflected EM wave with the direction of reflected wave propagation shown by the dashed hollow arrow 18.
In the conventional configuration of FIG. 1, the upper and lower halves of the dipole, 10a and 10b respectively, are equally excited by both the incoming EM energy and the reflected EM energy. However, because both EM waves are arriving at the dipole from an off-axis direction, the signal sensitivity of the dipole is reduced from what would be achieved if the waves were arriving broadside to the dipole.